The most extensively produced vehicle service brakes are those on automotive vehicles and trailers towed by automobiles or trucks. These brakes include drum brakes of two major types: leading-trailing, and duo-servo; disc brakes of two major types: fixed-caliper and sliding-caliper; various combinations of these major types; and other less-used types.
One typical combination of two of these types of brakes is that of a disc brake as the service brake, with the brake disc having a hub shaped much like the crown of a hat with the disc braking portion being the brim of the hat. A parking brake in the form of a drum brake is provided within the hat-shaped hub, with the interior cylindrical surface of that hub becoming the drum part of the drum parking brake. In this arrangement the drum brake is dedicated to parking brake functions.
Disc brakes are typically actuated as parking brakes by a threaded or cam device which causes the caliper piston or pistons, and/or the caliper housing itself, to move to engage the disc brake pads with the brake disc in braking relation. The threaded or cam device is usually actuated by an arcuately pivotable lever.
In other combinations, particularly of drum brakes, the parking brake arrangements are commonly integrated with the drum-type service brakes. The background which is most extensive is in the field of such combined parking and service drum brakes typically used on passenger vehicles and light trucks which have rear wheel drum brakes. Therefore, this description of the background of the invention is primarily directed to such brakes.
The parking brake mode of operation of most current drum brakes uses the same brake shoes and brake drum as are used for service braking. In the parking brake mode of operation, those shoes are typically moved by a pivoting internal lever or a cam, coupled to the service brake shoes so that arcuate pivoting movement of the lever or cam causes the shoes to be spread or expanded into braking engagement with the brake drum. A parking brake cable is typically secured to the lever or cam so that when tension force is exerted through the cable, the lever or cam is pivoted to actuate the parking brake.
It is well known to make vehicle service brakes which are actuated by use of a friction material on one end of a brake actuating arm, with an electromagnet also being on that arm end, and the other end of the arm being arranged to move the service brake shoes into engagement with a rotating drum or disc to slow or stop the vehicle when the electromagnet is energized. It is particularly common in the brake art, and especially concerning brakes for recreational vehicle and similar trailers, to use electrically actuated brakes of this type.
Typical patents disclosing electrically actuated brakes include U.S. Pat. Nos. 3,668,445--Grove, issued Jun. 6, 1972; and 3,760,909--Grove, issued Sep. 25, 1973. They have parking brake activation arms, each of which has friction material and an electromagnet on one end so that when the electromagnet is energized the friction material engages a rotatable armature face which rotates with the wheel to be braked. The resultant drag force causes the parking brake activation arm to pivot about its other end, and the brake shoes are spread apart by a cam or spreader bar arrangement on that arm end. Thus the brake shoes are engaged with the brake drum, and apply braking force to the rotating brake drum so long as the electromagnet is energized.